Paperboard sheets such as are used in the manufacture of cartons, boxes and other die cut products may be cut, trimmed, creased, perforated and/or otherwise "shaped" in a number of different ways. This may be done manually; by reciprocating flat dies; by sequentially subjecting each panel to the action of a plurality of discrete machines which respectively perform different ones of the desired cutting, slitting, perforating or other operations needed for desired shaping of the panel; or by use of a rotary die apparatus. When the number of sheets is sufficiently large, they can be most efficiently shaped by a rotary die apparatus. This is due to the high speed at which a rotary die apparatus operates, and also to its ability to substantially simultaneously perform a plurality of different shaping operations upon a sheet during single passage of it through the nip between cooperating die and anvil rolls of the apparatus. However, the prior art rotary die apparatuses have not heretofore produced tear strips, punch-outs, foldable creases, nicking connectors and similar weakened or "frangible" sections of optimum quality. Additionally, the prior rotary die apparatuses have not been able to form fold lines about which thereto adjacent panels of a paperboard sheet freely can undergo relative reverse pivotal movement, i.e., movement bringing the outer "printed" liners of the panels toward each other.
The metal die rule element customarily employed in a rotary die apparatus for forming tear strips, punch-outs, and similar perforate connectors has a plurality of relatively wide rectangularly shaped and laterally spaced teeth that extend outwardly from the base section of the rule and from the die board of the die roll. Except when the flat outer ends of the teeth have been recently sharpened, they do not easily or consistently penetrate the panels to the desired extent and tend to undesirably cut and/or crack the paperboard. When the frangible section of the panel is a tear strip, punch-out, nicking connector or the like, this makes it difficult or impossible for a person to readily and neatly tear the sheet at the desired frangible location(s). Another undesirable consequence of use of a perforating rule having rectangular teeth is the tendency of the paperboard sheet to adhere to the rule unless positively separated therefrom by product ejection means which may significantly reduce the strength of the sheet.
For certain types of paperboard containers, it is desirable to form the paperboard so that it has on one side serially aligned alternating slits and creases, and on the other side it has serially aligned discontinuous slits. The slits on the inner, or die-side, liner of the paperboard are of the same size and are directly opposite those on the outer ply. This type of pattern, known as perforation-scoring, is used to encourage folding of the paperboard at the line defined by the slits and creases. Typically, the paperboard is folded so that the portions of the inner liner on either side of the slit-crease line move to face one another, although the paperboard can also be folded in the opposite direction. The perforation-scoring pattern is formed by utilizing a die rule with a plurality of rectangular teeth, each of which is separated from adjacent teeth by a creasing edge. This rule has been used successfully with reciprocating flat-die processes in which the penetration depth of the rule can be closely controlled. When rules with rectangular teeth are used in rotary dies, however, the shortcomings described above for rectangular teeth adversely affect the consistency and quality of the slits and creases formed in the paperboard product.
In light of the foregoing, it is an object of the present invention to provide a die rule for perforating and creasing the inner ply of a paperboard and perforating the outer ply of the paperboard that can be used with rotary die processes without cracking or undesirably cutting the paperboard.
It is also an object of the present invention to provide a die rule that perforates and creases as described above that easily penetrates and releases the paperboard.
It is another object of the present invention to provide a method for perforating and creasing a paperboard as described above that uses a rotary die process.